High-resolution electron microscopy of nanocrystalline Ni-Al alloys: instability of ordered structure and dynamic behaviour of grain boundaries
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High-resolution transmission electron microscopy was performed on vacuum-deposited nanocrystalline nickel aluminide films. Several nickel aluminide ordered structures, i.e. L12(Ni3Al)-, B2(NiAl)-, D513(Ni2Al3)- and D020(NiAl3)-type structures, were observed in the deposited films. The L12 and B2 ordered structures became unstable with decreasing grain sizes. The critical grain size on transformation from the L12- and B2-type ordered structures into disordered structures was ca. 5 nm at ambient temperatures. High atomic diffusion, sufficient for grain growth, and an increase in the ordering occurred just above 400°C in the nanocrystalline Ni-Al films with L12- and B2-type structures. The diffusion bonding process, at ambient temperatures, between Ni-Al nanocrystallites with an L12-type structure was observed dynamically at atomic resolution under strong electron irradiation. It was found that the nanocrystallites rotated and slid without crack generation, and neck-growth proceeded even at ambient temperatures.
KeywordsGrain Size Transmission Electron Microscopy Ambient Temperature Ni2Al3 Bonding Process
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